the packer (and any large bore mouthpiece shank) should have an inner diameter of 13.5mm, or .530 inches.
conversely, any mouthpiece you use will come to a stop at the part of the shank where it is 13.5mm across.
the length that is sticks out, or the depth of penetration will vary from brand to brand. I have had a king euphonium that measured more than .530, and it was tricky finding a mouthpiece that worked with the shank. Bach and warburton were my only options
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